There is a high demand worldwide for materials that make it possible to regenerate the bone loss which cannot be self-regenerated by the human body. The human bone in 75% of its weight consists of nonorganic substance called bioapatite, which gives the stiffness to the bone and its resistance to mechanic injuries. The use of apatite from human bone material in a bigger scale meets a psychological barrier and carries the risk of transferring the donors' pathogens to the receiver. Therefore, for many years trials have been made to produce synthetic hydroxyapatite having a composition according to the formula Ca10(PO4)6(OH)2 and being an exact equivalent of bioapatite.
A key criterion decisive for the suitability of the material for implementology is the choice of method for consolidating the powder material in the final implant, which—on the one hand—should ensure the preservation of unique properties of powdered hydroxyapatite, such as: the grain size, phase composition and morphology, and at the same time make it possible to obtain the fittings having specific geometry and mechanical strength.
There are a lot of known methods for producing medical implants using mineral powders having a desired shape, strength, structure and content of physiologically active substances, which determine the acceleration or deceleration of biodegradation or resorption processes. Commonly the fixation of the suitable shape formed from powdered materials is based on the chemical binding of powders in a form of liquid cements solidifying under the influence of chemical or physiological factors. Such processes can be used both inside the body and at the stage of initial preparation of needed fittings. Another known solution is the solidification of the powder being a component of the slurry or paste, comprising an organic, natural or synthetic polymeric substance or prepolymer, which can also be hardened inside or outside the body. For the production of fittings for implantation in the body also conventional ceramic processes are used, comprising formation of a shape by casting or pressing and subsequent sintering of the formed fitting at a high temperature. Of course, such preparation of the implants is possible only outside the body.
The International Patent Publication Number WO 2005/074453 discloses the use of a paste containing calcium phosphates, other compounds containing calcium, phosphoric acid and other ingredients, to fill bone defects (especially for internal use) which hardens rapidly after application with the secretion of hydroxyapatite. A similar solution for a filling in a liquid form, solidifying in a presence of calcium sulfate hemihydrate, is disclosed in International Patent Publication WO 87/05521. In U.S. Pat. No. 7,258,735 circa forty inorganic substances suitable for solidification of this type of cement are disclosed.
U.S. Pat. No. 5,626,861 discloses a method for preparation of bone implants from a mixture of hydroxyapatite and polymers, and optionally additives of rinsable blowing agents in the form of an emulsion in a non-aqueous solution. The fittings obtained from the mixture are mechanically and structurally close to the natural bone.
In the embodiment disclosed in U.S. 2005/209704 the mineral material, for instance hydroxyapatite, in the form of granules is coated with a polymeric layer and by melting of the polymer layer and forming of the fittings biocompatible and biodegradable implants are produced.
U.S. Pat. No. 4,097,935 discloses a process for the production of fittings from hydroxyapatite which comprises the formation of hydroxyapatite, having a grain size from 0.2 to 3 microns (with possible additions of binding agents), into a particular shape and sintering at a temperature up to 1250° C. The effect of this process is a semitransparent ceramic material with a possible admixture of other phases of calcium phosphates.
U.S. Pat. No. 5,549,123 discloses the use of self-combustible agents whose function is to form a high temperature for sintering of the prepared fitting while sustaining inside the preferred porosity.
From PL 186129 is known a bioceramic implant made of a mixture comprising hydroxyapatite with other ingredients, inter alia: sulfur, soot and aluminum oxide, formed by molding at a temperature up to 200° C., casting, pressing or extrusion.
A technology is also known of consolidation of hydroxyapatite (HAp) as a nano-powder using isostatic pressing carried out at ambient temperature and a pressure of 4 kbar, which makes it possible to receive fittings having compressive strength up to 50 MPa without the structure and geometry of the material's grains change, characterized by the Young's modulus in a range from 0.8 to 2.2 GPa and a material density of 1.9 g/cm3 (D. Tadic, M. Epple, Biomaterials, 24 (2003), 4565-4571). However, it is known that the high temperature of isostatic pressing results in a phase transition of hydroxyapatite into calcium phosphate β (M A Auger et al. Ceramics International, 35 (2009), 2373-2380). A high-pressure, plasma spark sintering carried out at pressures up to 500 MPa and temperatures up to 1000° C. makes it possible obtain a transparent ceramics having a density up to 80% of the theoretical density and stable phase composition (M. Eriksson et al., Journal of the European Ceramic Society, 31 (2011), 1533-1540). Sintering of the powder by means of microwave energy at temperatures up to 1300° C. and a pressure of 200 MPa makes it possible to obtain a material of a 96-98% density, unchanged phase composition and grain size increased to 2.1 μm (S. Ramesh et al. Ceramics International, 33 (2007), 1363-1367).
A disadvantage of the known methods for HAp implants forming is the increase of the grain size and change in the phase composition of mineral material consolidated both by the reaction of cementation as well as the high temperature sintering. Limiting of the parameters of the known sintering processes in order to limit the growth effect of the particle size causes a low mechanical strength of fittings obtained with the density much lower than the theoretical density of the material.
In the production process of ceramic bone implants, having a degree of resorption adapted to osteogenic processes of the human body, it is preferred that the specific chemical composition and microstructure of the nanocrystalline HAp used for implants should not be altered during the process of consolidation (Meger J L et al. Inorganic Chemistry, 21 (1982), 3029-35; Dingreville R. et al., Journal of Physical Mechanical Solids, 53 (2005), 1827-1854). This condition is met when composites with polymeric compounds are used, however, in many applications, especially when exposed to mechanical stress, the resistance of composites of this type proves to be inadequate, because of their low mechanical strength and high biodegradability. Furthermore, the processes associated with the degradation of such structures often lead to severe inflammation.